I. Field of the invention:
The present invention relates to electro-erosion machines and more particularly to a device for separating from the machined workpiece the cutpart or "cutout" (hereinafter called clipping), produced by cutting a closed trajectory (or a trajectory whose ends reach the edge of the workpiece).
II. Description of the prior art:
Many solutions have been proposed to eliminate the machining defect which is caused by the clipping or cut part formed in a closed trajectory electro-erosion machining operation as it breaks away from the workpiece. The clipping will drop and then break off at some point just before the end of machining operations. Solutions to this problem have taken the form of a removable bracket located under the part to be machined JP-OS 57-126.932, 57-126.933 and 57-126.934; JP-OS 57-132.926, a center supporting part located under the latter (CH 656 083, or further, the provision of a supporting arm pivoting under the workpiece (JP-PS 58 1711218), or furthermore, of holding up of the dropping part by one of the supporting arms of the workpiece (CH 646892) by electromagnets (DE-OS 3.209136), and by using low fusion alloys or anaerobic glues (CH 559.599). However, where a connection is made between the workpiece and the clipping, these methods present a problem by interfering with accomplishing the finishing operation and by requiring that upon completion of the finishing operation it is still necessary to eliminate the binding material. In case of an additional supporting part being used to retain the clipping, there is generally a need to provide for a complicated controlling device to prevent an engagement between the wire electrode and the supporting arms and the additional supporting part. This is why it is recommended in the present state of the art, in particular in the Japanese patent application published under No. 58-28429, to stop the machining while the clipping is still being retained by a narrow bridge of material called the "isthmus".
For the sake of simplification, the word clipping will be used for the purpose of identifying the cut out part located inside the trajectory, whether it is a reject falling off the part to be obtained or the part itself desired to be obtained, the rejected portion being part of the rough-machined part. The mechanical resistance of the isthmus must be sufficient to counterbalance the mass of the clipping and to prevent it from collapsing and falling which can cause damage to the wire electrode or cause it to deviate the wire electrode to produce uneraseable marks on the machined surface. This isthmus can exist only on a part of the height of the clipping. Next, the clipping is broken off by hitting it with a hammer or cutting out the isthmus with a contour saw, as described in the Japanese application published under No. 58-28429. It is thus possible to make one or several rough-turnings on the same part, but it requires the intervention of an operator to hollow the cut out contours and, eventually, to reposition the part in the machining area before starting the finishing operation. A finishing operation could be, in fact, required to eliminate the "scar" actually formed because of the break of the isthmus and done through electro-erosion process or by any other known method.
According to the Japanese patent application published under No. 60-259323, the clipping can be also beaten down with an upper machining liquid supply nozzle for the purpose of breaking the isthmus (located on the whole height of the part to be hollowed) and then held up by a bottom machining liquid supply nozzle; the latter is then operated vertically upwards in order to remove the clipping for the hollowed orifice above the upper surface of the machined part. At this instant, an auxiliary supply emits a jet of machining liquid in order to blow out the clipping from the machining zone.
However, this arrangement can only be used for the purpose of separating clippings of a certain shape and it requires special devices on the two machining heads, as well as several precise adjustments of the latter. In fact, the machine must be provided with a bottom head capable of being moved vertically. Furthermore, since the upper nozzle is activated hydraulically downwards until reaching a fixed stop, its vertical displacement is fixed. It is therefore necessary to precisely adjust the distance between the clipping to be ejected and the upper head. Another inconvenience is the free working of the clipping in the hollowed hole. If the pressure produced by the lower nozzle, for the purpose of raising it across the hole is not applied exactly on its center of gravity, the clipping will get stuck against the rim of the hole, especially against the protuberance formed by the isthmus of material which subsides on the total height of the hole. The geometry of the ejectable clippings, using this method, is therefore limited to clippings which are not too thick, especially having a flat and horizontal lower face and horizontal, as well as an upper face whose center has a flat surface parallel to the lower face and whose surface is at least equal to the impacting surface of the nozzle. It must be mentioned that in case of machining several holes simultaneously in the workpiece, the clearing of the clippings by means of a liquid jet risks having the clipping getting stuck in one of the holes which have been machined already and is located on its trajectory.